1. Industrial useful field
This invention relates to a clutch cover assembly equipped with a coned disc spring for controlling an increase in a releasing load due to wear of the facings of a clutch disc.
2. Prior art and its problem
FIG. 12 shows a conventional clutch cover assembly (Japanese Unexamined Utility Model Publication No. 64-24732), in which a diaphragm spring is held by a pair of wire rings secured to bent tabs of a clutch cover and equipped with a coned disc spring for controlling an increase in releasing load due to wear of facings of a clutch disc. In this clutch cover assembly, facings 52a and 52b are provided on both outer peripheral surfaces of a clutch disc 51, and the facing 52a is pressed on a flywheel 54 by a pressure plate 53. An outer peripheral part of the pressure plate 53 is covered by a clutch cover 55. An annular diaphragm spring 56 is disposed between the clutch cover 55 and the pressure plate 53. Approximately L-shaped bent tabs 55a protrude integrally from an inner peripheral edge of the clutch cover 55 with appropriate spaces in circumferential direction left therebetween. Bent tabs 55a pass through enlarged openings 56b formed on ends of slits 56a of the diaphragm spring 56. The bent tabs 56a support inner peripheries of three wire rings 58a, 58b and 58c. The diaphragm spring 56, at its radial intermediate part, is held by wire rings 58a and 58b. The diaphragm spring 56 is pressed on a fulcrum land 53a on a backside of pressure plate 53 at its outer peripheral part and contacts with a release bearing (not shown) of a releasing means at its inner peripheral part. A coned disc spring 59 is disposed between the diaphragm spring 56 and the pressure plate 53 and is held by the wire rings 58b and 58c at its inner peripheral part. An outer peripheral edge of the coned disc spring 59 is bent to contact, at its top end, with the diaphragm spring 56. Plural concave portions 59a fitting onto the bent tabs 55a are formed on an inner peripheral edge of the coned disc spring 59 with appropriate spaces, in circumferential direction left therebetween.
Generally, the diaphragm spring 56 has such a characteristic that its deflection load increases when its deflection decreases from some value to a specified value and the deflection load decreases when the deflection further decreases. For this reason, when the facings 52a and 52b are worn out at time of clutch engagement, a deflection load of the diaphragm spring 56 increases with a decrease in a deflection of the diaphragm spring 56. Consequently, a pressing force exerted from the diaphragm spring 56 on the pressure plate 53 increases so that a required treading force of clutch pedal also increases. Therefore, the annular coned disc spring 59 is installed between the diaphragm spring 56 and the pressure plate 53 so as to offset the increase in deflection load of the diaphragm spring 59 by means of the deflection load of the coned disc spring 59.
Accordingly, even when the facings 52a and 52b are worn out to cause the deflection load of the diaphragm spring 56 to increase at time of clutch engagement, the deflection load of the coned disc spring 59 is thereby increased to urge the outer peripheral part of the diaphragm spring 56 toward the clutch cover 55, so that the increase in the pressing force exerted from the diaphragm spring 56 on the pressure plate 53 is controlled.
In the above-mentioned conventional clutch cover assembly, however, its assembly work was very laborious because the three wire rings 58a, 58b and 58c had to be contacted with the bent tabs 55a at their outsides, and moreover had to be installed so as to hold the diaphragm spring 56 and the coned disc spring 59 from both sides. Further, because the three wire rings 58a, 58b and 58c were put side by side in the axial direction, it was difficult to shorten the axial length of the clutch cover assembly.
FIG. 13 is a vertical sectional partial view showing another conventional clutch cover assembly equipped with a coned disc spring. In this figure, symbols the same as those of FIG. 12 represent the same or equivalent parts. 60 is a stud pin which is installed at plural places of clutch radial inner peripheral part of the clutch cover 55 with spaces left therebetween in circumferential direction. The stud pin 60 is so installed as to protrude in axial direction toward the pressure plate side through the enlarged opening 56b formed at the clutch radial outer peripheral end of the slit 56a of the diaphragm spring 56. Inner peripheries of the two wire rings 58a and 58b are supported by the stud pins 60. 61 is an annular coned disc spring. The coned disc spring 61 is installed so that its radially outer peripheral edge makes contact, from the diaphragm spring side, with a stepped part 55b of the clutch cover 55 located at a position radially inwardly than the stud pin 60 and is secured thereto by a clip (not shown) etc. The radially inner peripheral edge of the coned disc spring 61 makes contact with diaphragm spring 56.
According to this structure, even when a facing 52 is worn out to cause the deflection load of the diaphragm spring 56 to increase at time of clutch engagement, the deflection load of the coned disc spring 61 is thereby increased to urge an inner peripheral part 56c of the diaphragm spring 56 toward the pressure plate in axial direction, so that its outer peripheral part 56d is urged toward the clutch cover in axial direction. Consequently, the increase in pressing force exerted from the diaphragm spring 56 onto the pressure plate 53 is controlled.
FIG. 14 shows deflection/load characteristics of the diaphragm spring 56 and the coned disc spring 61 of the embodiment of FIG. 13. An arrow in parallel with an axis of abscissa indicates an increasing direction of deflection of the diaphragm spring 56 and a decreasing direction of deflection of the coned disc spring 61, respectively. PS represents a pressing load characteristic of the diaphragm spring 56 on the pressure plate 53, and PL represents a characteristic of load required when releasing the diaphragm spring 56 by a release bearing mechanism i.e. a releasing load characteristic. C1 and C2 represent load characteristics of the coned disc spring 61. As described above, the increase in pressing load of the diaphragm spring 56 on the pressure plate 53 is offset by the deflection load of the coned disc spring 61. Namely, PS is offset by C1 to be changed to a curve as shown by a broken line A.
Paying attention to the releasing load characteristic PL, however, when releasing the diaphragm spring 56 by pushing its inner peripheral edge toward the flywheel in an axial direction, a clearance is produced between the inner peripheral part 56c and the inner peripheral edge of the coned disc spring 61 due to the deflection of the inner peripheral part 56c of the diaphragm spring 56. Thus, the coned disc spring 61 works later than PS by a time corresponding to L1, FIG. 14. Namely, PL is offset by C2, changed to a curve as shown by a broken line B. Accordingly, this structure included such disadvantage that the effect of decreasing the releasing load i.e. the required treading force of clutch pedal was minimized. Further, in the clutch cover assembly having the foregoing structure, the coned disc spring 61, having a deflection/load characteristic in which the deflection load does not become zero even after it increases and then decreases, is used in order to avoid a phenomenon where the coned disc spring 61 springs back, due to excessive deflection, and does not operate thereafter. That is, the load does not become zero at a portion X as shown by C1 and C2, FIG. 14, in the coned disc spring 61 For this reason, a wear allowance (W1 in the figure) when using the coned disc 61 became smaller than a wear allowance (W2 in the figure) when not using the coned disc spring 61, shortening the service life of the clutch.